A unique combustion process, referred to by various generic names such as premixed charge compression ignition (PCCI), homogeneous charge compression ignition (HCCI), or controlled auto-ignition (CAI), has been the subject of intensive research because of potential advantages of high efficiency, near-zero NOx emission and extremely low smoke pollution. It is also a common-mode combustion process which could be used advantageously in both types of the conventional internal combustion engine, namely, the gasoline spark ignition (SI) engine and the diesel compression ignition (CI) engine. However, there are several technical problems related to the use of the above combustion process in automotive applications. Firstly, it is difficult to control, for lack of direct triggering means, precisely the start of CAI/HCCI combustion which tends to happen on its own depending on the past temperature history of the premixed fuel/air mixture. Secondly, the relatively small part load operating range of CAI/HCCI combustion makes it necessary for the engine to be switchable instantaneously between the conventional SI or CI combustion mode and the CAI/HCCI combustion mode in order to achieve full range engine operation. It is difficult to achieve seamless switching to CAI/HCCI mode, which may take place under a wide variety of engine operating conditions prevailing at the time of switching, without knowing the actual auto-ignition timing and what corrective action might be necessary to move it to an optimum timing.
It is known that auto-ignition combustion and the precise timing of the auto-ignition are influenced indirectly but strongly by a large number of internal and external parameters, including engine compression ratio, engine speed and load, fuel composition, fuel/air mixture ratio, fuel injection and evaporation, intake charge quantity and temperature, coolant temperature, EGR quantity and temperature, residual charge quantity and temperature etc. Change in any one of these parameters could induce auto-ignition and alter the auto-ignition timing, while changes in several of these parameter could interact with one another, alter the auto-ignition tendency and influence the auto-ignition timing in many ways. It is necessary to understand the effect of each of the parameters and describe them quantitatively in a measurable and calibratable manner so that a control system may be designed to take into account all the changes and command the precise corrective action explicitly in response to an auto-ignition timing demand. This is however an extremely difficult task because of the myriad of the above indirect but strongly influencing parameters leading to an exponential explosion of the calibration effort as well as unmanageable increase in the complexity of the control system.
For example, the effective compression ratio of the engine may be chosen as the prescribed engine operating parameter (in preference to some other equally effective parameters) for inducing auto-ignition and influencing the auto-ignition timing as proposed in U.S. Pat. No. 6,427,643. In this case, although the directional requirements for the change in compression ratio are known, such as higher compression ratio for lower loads, lower compression ratio for higher intake air temperature etc, it is not possible to define a precise command setting of the compression ratio explicitly that would directly trigger a predictable and precise auto-ignition timing under any operating condition according to a predetermined auto-ignition timing map while the rest of the above mentioned parameters are all having influence and must be accounted for in a measurable and calibratable manner. Unlike a spark ignition engine where a precise command of the spark timing will directly trigger a predictable and precise timing of ignition, or a diesel engine where a precise command of the fuel injection timing will directly trigger a predictable and precise timing of flame initiation, both involving substantial calibration effort and control complexity, there is no equivalent direct trigger in the auto-ignition engine. Changing the compression ratio according to a compression ratio map is not sufficient to guarantee that the auto-ignition timing will match a higher level map of target auto-ignition timing with sufficient accuracy that is expected in a modern controlled and optimised engine. As mentioned earlier, the technical barrier remains because of the myriad of indirect but strongly influencing parameters in addition to compression ratio leading to exponential calibration effort and unmanageable control complexity.
US2003/0097998 proposed a method that attempts to directly trigger the auto-ignition timing by introducing a sudden increase in compression ratio near the end of the normal compression stroke of the engine. In this case the auto-ignition is confined within a narrower timing window close to TDC of the engine, but the precise auto-ignition timing would still depend on the magnitude of the sudden compression ratio increase. Of course, a very large sudden compression ratio increase will immediately trigger auto-ignition by brute force but at the expense of very high combustion pressures that would immediately follow leading to excessive stress, noise and high NOx emissions. Providing a measured sudden compression ratio increase would be the desired solution, but this would still require huge calibration effort and control complexity in taking into account all the above mentioned indirect but strongly influencing parameters in order to be able to define a precise command setting of the sudden compression ratio increase that would produce a predictable and precise auto-ignition timing under any engine operating condition according to a predetermined auto-ignition timing map.
Ignition timing is commonly defined as the time when the instruments detect 5% of the charge has ignited and burnt. Ignition delay is commonly defined as the delay time from the time of activation of the charge for initiating combustion to the above detected ignition timing. The rate of combustion will increase rapidly starting from the ignition timing, and later slow down towards the end of combustion which is commonly defined as the time when the instruments detect 95% of the charge has been burnt.
The term calibration as used herein relates to obtaining by prior measurements, theoretical calculations, or a combination thereof, a set of values of a plurality of auto-ignition timing points for a variety of engine settings and operating conditions. Such mapping of timing points, and storage of the optimum values into a finalised calibration map (or look-up table) will define the target auto-ignition timing and the associated delay duration for the engine, when the engine (or a similar engine) is subsequently controlled according to the present invention to operate at the same optimum condition.